That’s not literally correct. In fact, it’s off by 180 degrees. Because the chapel’s altar is built on the western wall, the change meant the pope was facing west during much of the liturgy.

On the contrary, “ad orientem” was the direction popes faced when they used the free-standing portable altar in the Sistine: the celebrant faced east when he faced the people.

I spoke the other day to Msgr. Enrico Vigano, who has worked many years in the Vatican’s liturgical office, and he agreed that the term “ad orientem” doesn’t make sense in the Sistine Chapel.

Instead, he said, his office made reference to the cross, which stood on the main altar, framed by Michelangelo’s fresco of the Last Judgment. The idea here — and it’s one Pope Benedict has made in the past — is that when the pope and the people face the cross together, it emphasizes that the Mass is a common act of worship.

In early Christian churches, facing the cross coincided with facing east, the direction of the rising sun and, in a figurative sense, of the resurrection and the second coming.

But the Christian tradition of worshipping “ad orientem” faded, and over the last 500 years many churches have been built facing different directions, including one not far from the Sistine Chapel — St. Peter’s Basilica, which also faces west.

Whatever a church’s compass orientation, some have wondered whether the papal Mass last Sunday marked the beginning of a trend. Are we going to see a Vatican effort to turn all the altars back to the pre-Vatican II position?